Textile fabrics and felts for technical



3,8@,Zl8 Patented May 14, 1963 3,089,218 TEXTTLE FABRICS AND FELTS FQRTECHNICAL PURPQSES Glof Snnden, Ljungaverl; Sweden, assignor toStochholms Superfcsfat Fabrilrs Alrtiebolag, Stockholm, Sweden, acorporation of Weden No Drawing. Filed June 2, i958, Ser. No. 755 ,6923

16 Claims. (ill. 28--78) This invention relates to textile fabrics andfelts for technical purposes consisting of polyacrylonitrile fibers withimproved tensile strength, modulus and resistance against acidichydrolysis, particularly at higher temperatures.

More particularly the invention relates to papermakers felt for use inthe press section and the dryer section of paper machines, such asFourdrinier machines.

In manufacturing of paper, paperboard and other board productspaperrnakers felts are used for carrying and for dewatering and dryingthe sheet and moreover to secure a desired sheet surface. These feltsare usually prepared as endless belts or belts joined together toendless belts.

Such felts have hitherto for the most part been prepared from twistedwool yarn and woven in a special manner and milled to obtain the desireddimensions.

Owing to friction, abrasion, chemical and bacterial deterioration, suchfelts have a limited period of use, owing to the fact that they oftenhave to be washed and substituted. Efforts have therefore been made toincrease the period of use by adding synthetic resins to the felt, forexample phenolformaldehyde resins or alkylated melamine resins, but thenit is dilficult to obtain a satisfactory curing of the resin withoutdamaging the temperature sensitive woolen fiber-s. Cotton has also beenused as material for papermakers felt. Since the synthetic fibers wereintroduced on the market, many efforts have been made to use such fibersin these felts. The reason for these attempts has been the defectivewearing strength of the natural fibers and their property to swell inmoisture and heat, thereby given a thicker and non-permeable felt. 'Ihus polyamide fibers have been used as reinforcing materials in woolenwet-felts, while polyesters commonly have been used in dry-end felts.

The most significant disadvantage of the natural fibers as well as ofthe synthetic fibers hitherto used is their bad chemical resistanceagainst deterioration of the polymer structure and the fiber in the hotand damp conditions of a paper making machine. The deterioration ofwool, cotton, polyamide and polyester fibers moreover seems to becomeremarkably aggravated in the slightly acid conditions caused by thepresence of aluminum sulfate in the paper manufacture.

When the acrylonitrile fibers appeared, their outstanding propertieswere recognized in respect to resistance against degradation bysunlight, mildew, bacteria and acid hydrolysis, and these propertiesseemed to predestinate this fiber-group for a wide use in the technicalfield. The practical results were, however, not very successful andtoday, acrylic fibers have found some limited use in the technicaltextiles field. The main reason for this lack of success has probablybeen their poor strength and modulus of elasticity at high temperatures,thus causing creep-effects at high temperatures. Their abrasionresistance and flex life, which are not too good compared with polyamideand polyester fibers but mostly better corn pared with wool and cotton,also limit their applicability in the technical field.

The main drawback of the earlier acrylonitrile fiber from a pure textilepoint of view was the dyeing difliculties and the fibrillation tendency(fiber-splitting into smaller fibers). To solve these problems, theacrylic fibers were developed in such a direction as to make dyeablecopolymers of different compositions and to attain a rather loworientation of the fibers. Consequently the mechanical properties andparticularly the creep at high temperatures became still moreundesirable. Also the resistance against acidic degradation wasimpaired.

THE INVENTION BROADLY In accordance with this invention textile fabricsand felts for technical purposes are prepared from fibers or yarns ofslightly inter-linked acrylonitrile copolymers. More specifically theacrylonitrile copolymer used should contain at least molar percent ofacrylonitrile units and from 0 to about 10 molar percent ofmonoethylenically unsaturated monomer units copolymerizable withacrylonitrile, inter-linked to a degree of one inter-link per from 2 to12 polymeric chains by means of an interlinking polyfunctional compound.Said degree of interlinking or cross linking may also be defined as 1interlink per 1000 to 20,000 monomer units in the polymer, whichcorresponds with the formation of centrally branched polymer moleculeswith up to six polymeric chains radiating from the inter-linking centralpoint. This type of polymeric structure has been named multiehainmolecules by Flory et al. in respect of polycaprolactams. Theseradiating polymeric chains can orient independently and build up fiberforming bonds between different multi-chain molecules.

These inter-linked copolymers result in fibers with improved mechanicaland elastical properties and an improved creep resistance at highertemperatures as to C. and therefore the range of uses of fibers of thiskind may be considerably broader than fibers made from linearcopolymers.

The monoethylenically unsaturated monomer preferred in the acrylonitrilecopolymer for this purpose is selected from the group consisting ofvinyl acetate, acrylic acid, acrylamide, methacrylonitrile,methacrylamide and an ester of acrylic acid and methacrylic acid.

The inter-linking polyfunctional compound can be a compound of differentcharacters. The inter-linking compound can be a diethylenieally ortriethylenically unsaturated monomer or a salt bridge formed by apolyvalent base or acid, such as a poly'valent metal. As examples 1'diethylenically unsaturated monomers divinyl benzene,methylene-bis-acrylamide, diallylphthalate, diallylmaleate and ethylenediacrylate (dimers) are mentioned. As an example of a triethylenicallyunsaturated monomer triacrylylperhydrotriazine (trimer) is stated. Theinterlinking by a polyethylenically unsaturated compound is performedduring the polymerization. The inter-linking by polyvalent metals,however, is preferably performed by an after-treatment of a copolymercontaining acid groups or a fiber containing said copolymer.

The content of inter-linking compound should be determined for eachinter-linking compound, paying due attention to the requirement of thepolymerization process used and the relative reaction rate ofacrylonitrile with the agent. We have preferably employed solution oremulsion polymerization in water with water-soluble catalysts. The upperconcentration limit has been determined by the necessity of the polymerto be completely soluble in the spinning solvent and of keeping theviscosity of a spinning solution of normal concentration (about 1820%)low enough to permit its technical filtration and de-aeration. In viewor" what is said above, the suitable concentration ranges of someinter-linkers have been determined for some suitable inter-linkers. Thefigures of Table I are stated in moles of inter-linker per 1,000 molesof acrylonitrile. They furthermore relate to a viscosimetric molecularweight of the polymer of about 70,000 calculated from the Staudingerformula.

3 Table I [01 inter-linker per 1000 units acrylonitrile Divinyl benzeneAbout 1. Methylene-bis-acrylamide 0.2-0.6, preferably 0.35.Triacrylylperhydrotriazine 0.05-02, preferably 0.12. Diallylphthalate0.10.2. Diallylmaleate 0.1-0.8.

As specific examples of the inter-linked copolymer the followingcompositions are mentioned:

(i) A copolyrner prepared from 97 kg. acrylonitrile and 3 kg. acrylicacid inter-linked by 120 g. methylenebisacrylamide and (ii) A copolymerprepared from 95 kg. acrylonitrile and 5 kg. methylacrylate inter-linkedby 60 g. triacrylylperhydrotriazine.

The molecular weight range for fiber forming purposes is from about30,000 to 100,000, preferably between 40,000 and 90,000, the latterfigure corresponding to an intrinsic viscosity of 140 respectively 300ml./ g. measured in dimethyl formamide solution. In practice theinterlinkers rnethylene-bis-acrylamide and triacrylylperhydrotriazineare to be preferred as they involve no hydrolyzable inter-links.

For preparing fibers from copolymers of the kind described thecopolyrner is dissolved in an appropriate polymer solvent such asdimethyl formamide, dimethyl acet amide, dimethyl sulfoxide, ethylenecarbonate and propylene carbonate. The spinning solution is extrudedinto a bath that is miscible with the polymer solvent but precipitatesthe polymer in its filament form. As coagulants water, aqueoussolutions, alcohols, such as glycerine, aromatic hydrocarbons, such asbenzene and cymene, may be used. The solutions of the inter-linkedpolymers according to this invention may advantageously be extruded invery slow acting coagulating baths, such as liquid hydrocarbonspredominantly consisting of parafiinic hydrocarbons, such as commercialparaifinic kerosenes with a boiling range of ISO-250 C. Solutions of thesaid interlinked polymers or copolymers may also be extruded into aheated spinning cell in accordance with the common dry-spinningtechnique. Obviously, mixtures of the said inter-linked copolymers, andother polymers or copolymers may be used for forming filaments asstated. The various spinning techniques are more completely described invarious US. patents, e.g. in Patents Nos. 2,404,714 and 2,404,715, andthe spinning into liquid hydrocarbon mixtures in the copending patentapplication Serial Number 327,429, filed Dec. 22, 1952, now abandoned,Serial Number 662,316, filed Aug. 29, 1957, now Patent No. 2,967,085,and Serial Number 662,352, filed May 29, 1957, now Patent No. 2,967,086.

The specific structure of the multi-chain acrylonitrile polymers givesthe fiber not only a high resistance to creep but also a higher modulusand a higher strength compared with linear acrylonitrile fibers with thesame degree of orientation, while the strain properties are un 4aifected. The inulti-chain acrylonitrile fibers have a much betterbreaking tenacity than linear acrylonitrile fibers. The dimensionalstability of the multi-chain acrylonitrile fibers is also higher thanthat of linear acrylonitrile fibers when the material is relaxedsatisfactorily after yarn manufacturing and weaving.

The multi-chain acrylonitrile fibers give fabrics with a very highdimensional stability and extremely high resistance to yarn-slippage incontrast to most synthetic fibers. This stability gives the felts aquiet running even at very high speeds. These properties combined withthe extremely high bulk of the fiber makes it possible to make dryendfelts in the lightest weights with retained dimensional and dynamicstability and also high pliability.

Like linear acrylonitrile fibers, the multi-chain acrylonitrile fibersundergo a very slow hydrolysis under hot acidic aqueous conditions. Thishydrolysis, however, is so slow that it does not afiect the propertiesduring years of service, but on the contrary, the hydrolysis involves afurther molecular inter-linking of the multi-chain acrylonitrilecopolymers containing acid or ester groups. polyacrylonitrile does notshow as great a subsequent molecular inter-linking. Fibers containingfree acid groups seem to be hydrolysed too fast and can be destroyed.The best results therefore have been achieved by copolymers containingesters of acrylic acid and methacrylic acid, for example methylacrylate.The rate of further interlinking is much higher for the multi-chainmolecular structures than for the linear molecular structures. Thisfurther inter-linking of the molecules gives an insignificant or nochange in the room temperature properties, such as strength andelongation. Table 'IiI shows some results from treatment of difierentfibers in boiling Water and aluminum sulfate solutions. By the acidictreatment the increase in high temperature strength is much more rapidin the multi-chain fibers containing methylacrylate than in the linearcopolymers. The 100% acrylonitrile polymer does not show any increaseabove its original high temperature strength after the acid treatment.

Table III [Resistance of different acrylom'trile fibers to boiling waterand boiling solutions of aluminum sulfate. (Samples boiled in glass withreflux and tested in an Instron apparatus)] Initial Resulting strength,Initial strength, Resulting Flber Treatment Time, g./d., elong.strength, g./d., elong. strength,

hours percent, g./d. percent, g./d.,

6. 150 0. 65% RH 65% RH Linear aerylonitrile fiber with {Water 1, 000 2.3/25 0.25 1. 9/23 0.25 M5l6%r11]1ethacrylate(diiygipjun). 5% lMSOOs 1,3/2.; 0.25 1. 9/24 0.60 u ti-e ain acry onitri e ers: a er .9 2 0.74.2/28 0. 7 gstywet splim) lmiZfscyloniglilel. 112(SO4)3.. 11., 5/24 0.24. 2/29 0. 7

0 acry oni ri e me y a a er .6 30 0. 5 3.2/29 0.75 g ayerylatei t 1 +3;1 {5% A1z(SO4)a- 1, 0(2)? 3.1/37 2.50 7 acry oni ri e acry ie a 1 3.3343 0.7

azid. }5% A1z(s0i)z i 168 3. 4 31 0. 60 3. 2/ 1. 3

The multi-chain acrylonitrile fibers of this table are inter-linked by0.050.06% of a trimeric inter-linker (triacrylylperhydrotriazine).

The further inter-linking substantially influences the strength andelongation at higher temperatures, such as 150 C., and accordingly theplastic deformation (the plastic flow) of the fibers decreases withincreasing molecular inter-linking. An X-ray investigation has shownthat no significant change of the crystallinity of the fibers occurs asa result of the further inter-linking. An infrared analysis, however,indicates an increase in ionic carboxylic groups capable ofinter-linking by polyvalent metals, such as aluminum, magnesium, nickeland bivalent copper. The polyvalent metal atom is thereby chemicallybonded between two molecules.

Such salt bridges also can be used as the original interlinking units inthe fiber, for instance by treating fibers containing acrylic acidgroups with aluminum sulfate. The inter-linking by polymerization,however, is to be preferred.

An inter-linked acrylonitrile copolymer containing 97 parts ofaerylonitrile, 3 parts of methylacrylate and 0.05 part oftriacrylylperhydrotriazine prepared by co-polymerization have valuableproperties for papermakers felt, but the valuable properties can befurther improved by further inter-linking by means of aluminum sulfateor other acidic substances in the pH-range of 26 in the aqueous medium,where the fiber is treated and boiled.

The valuable influence of the acidic treatment on the high temperatureproperties has its maximum at about pH 2. Below pH 1 and above pH 8, themulti-chain acrylonitrile fibers begin to lose their high temperaturestrength, as well as their room temperature strength, but

these specific pH-conditions have no influence in the ordinary papermaking.

The fabrics and felts of the inter-linked copolymers of this inventionalso have valuable properties in respect of water adsorption caused bythe strong capillary forces. 0

Of all known fibers the acrylonitrile fibers have the highest rate ofwetting-n property of great importance both for dryand wet-end felts inpaper machines.

The fabrics and felts have also valuable drying properties in that thewater evaporates exceptionally fast. The drying conditions in a papermachine also involve transfer of heat from the drying cylinders to thefelt and the paper sheet. As there is little difference in the heatconductivity between different dry fibers, the fiber which has thefastest distribution for water practically will have the best heatconductivity.

Evidently the drying characteristic of the slightly interlinked fibersdepends on its capillary properties, which causes both a rapid wettingin contact with the damp sheet and a rapid constant rate of drying incontact with the heated drying cylinders.

There is nothing particular about the construction of the papermakersfelt itself. The felt has the form of an endless belt comprising a wovenfelt base consisting of warp yarns and weft yarns of fibers of theslightly interlinked polyacrylonitrile fibers or filaments. Obviouslythe specific acrylonitrile fibers can be blended with other fibers, suchas wool and cotton or other synthetic fibers when particular effects aredesired. This is not recommended, when the best over-all properties areimportant.

A number of dry-end felts of 100% inter-linked acrylonitrile fibers of 3denier have been operated in paper machines for experimental purposes.They have run very satisfactorily and after two years of experimentaloperation the fibers exhibited the same strength and the same abrasionalresistance as the original fibers. An increase in high temperaturestrength has been evident.

The improved textile fabrics and felts of this invention mayadvantageously be used also as filter medium in filter presses and oncontinuous filters, such as rotary filters, operating at highertemperatures, particularly when acidic media are treated. Furthermorethe increased high tem perature strength is important for such uses ascordage and tire cords.

EXAMPLE 1 Two different copolymers were prepared from 97 kg.acrylonitrile, 3 kg. acrylic acid and g. methylene-bis- -acrylamiderespectively, 95 kg. acrylonitrile, 5 kg. methylacrylate, 60 g.triacrylyperhydrotriazine in the following manner. The monomer mixturewas gently poured during 3 hours into 400 1. water at 50 to 55 C.containing dissolved l g. ammoniumpersulfate, 1.5 sodiumpyrosulfite andl g. sodiumlaurylalcoholsulfate per liter. The polymerization wascontinued for 4 hours and then a yield of 95 kg. precipitated and driedpolymer was obtained. The polymers had a molecular weight of60,000-65,000 as calculated from viscosity measurement by the Staudingerequation (see page 967, volume 10 of The Encyclopedia of ChemicalTechnology, by Kirk- Othmer).

EXAMPLE 2 An 18 percent solution in dirnethylformamide was prepared froma copolymer of Example 1 and extruded without any preheating through a1000 hole-spinneret, hole-diameter 0.15 mm. with a velocity of 250 ml.per minute. The spinneret was arranged in the bottom of a verticalstem-mended tube of 3 m. length, through which an aromatic free kerosene(boiling range 160-200 C.) with a temperature of C. was running fromabove to the bottom (counter-flow). The peripheric velocity of thecollecting godet in the upper part of the tube was 25 m. per minute.After said godet the fiber was stretched 6 times its original length at130 C. to another godet with the peripheric speed of m. per minute.After relaxation at 130 C. in air and crimping, the fiber was cut tostaple fibers. The fiber was washed with boiling water containingnonionic soap (a polyalkylene oxide) during 30 minutes at pH 4 to removethe content of dimethylformamide and kerosene, rinsed, treated with acatonic agent, Querton 18 AST (an octadecyl dimethyl ethyl ammoniumethyl sulfate), and dried in air at 120 C. The fiber had a tenacity of3.0 denier (measured microscopically 3.1 to 3.2), a tensile strength of3.5 g. per denier and an elongation at rupture of about 35 percent. Thed-ye receptivity to basic dyes, for example Du Pont Basic Blue, wasexcellent. The amount of dye saturation was about 10 percent dye in thefiber for the acrylic acid containing fiber and 8 percent for themethylacrylate containing fiber. The fiber containing acrylic acid wasmore sensitive to heat and boiling in alkaline solution than the fibercontaining methylacrylate.

EXAMPLE 3 Papermakers dry end acrylic felts have been woven from fibersspun according to Example 2, and installed in the drying section ofdifferent paper-machines.

In a machine, where woolen felts of twice the weight lasted for 46months, the acrylic felt runs satisfactorily even after 2.4 months use.Fibers taken from the felt at that time had the same strength as theyhad in the new felt.

In a fast moving machine, where woolen felts lasted for about 2 monthsonly, there was no change in appearance or functionality of the acrylicfelt after 8 months use.

I claim:

1. A papermakers felt comprising a woven cloth in which the warp and thefilling are composed essentially of a yarn prepared from the fibers ofan acrylonitrile copolymer containing at least 90 molar percent ofacrylonitrile units and up to about 10 molar percent ofmonoethylenically unsaturated monomer units, inter-linked to a degree ofone inter-link per 1000 to 20,000 monomer units in the polymer by meansof an inter-linking polyfunctional compound.

2. A papermakers felt according to claim 1, wherein themonoethylenically unsaturated monomer is selected from the groupconsisting of vinyl acetate, acrylic acid,

acrylamide, methacrylonitrile, methacrylamide and an ester of acrylicacid and methacrylic acid.

3. A papermakers felt according to claim 1, wherein the inter-linkingpolyfunctional compound is a polyethylenically unsaturated monomer.

4. A papermaker s felt according to claim 3, wherein the inter-linkingcompound is a diethylenically unsaturated monomer selected from thegroup consisting of divinyl benzene, methylene-bis-acrylamide,diallylphthalate, diallylmaleate, ethylene acrylate and ethylenediacrylate.

5. A papermakers felt according to ciaim 1 wherein the inter-linkingcompound is a triethylenically unsaturated monomer copolymerizable withacrylonitrile.

6. A papermakers felt according to claim 5 wherein the inter-linkingcompound is triacrylylperhydrotriazine.

7. A papermakers felt according to claim 1 wherein the inter-linkingpolyfunctional compound is a polyvalent metal forming salt bridgesbetween acidic groups of the polymer.

References Cited in the file of this patent UNITED STATES PATENTS2,426,728 DAlelio Sept. 2, 1947 2,581,790 Gates Ian. 8, 1952 2,821,458Evans Jan. 28, 1958 2,821,771 Skeer Feb. 4, 1958 2,861,051 Caldwell Nov.18, 1958 2,883,371 Thomas et al. Apr. 21, 1959

1. A PAPERMAKERS'' FELT COMPRISING A WOVEN CLOTH IN WHICH THE WARP ANDTHE FILLING ARE COMPOSED ESSENTIALLY OF A YARN PREPARED FROM THE FIBERSOF AN ACRULONITRILE COPOLYMER CONTAINING AT LEAST 90 MOLAR PERCENT OFACRYLONGITRILE UNITS AND UP TO ABOUT 10 MOLAR PERCENT OFMONOETHYLENICALLY UNSATURATED MONOMER UNITS, INTER-LINKED TO A DEGREE OFONE INTER-LINK PER 1000 TO 20,000 MONOMER UNITS IN THE POLYMER BY MEANSOF AN INTER-LINKING POLYFUNCTIONAL COMPOUND.